As an engine configured to combust reformed fuel as described above, there is known an engine disclosed in Japanese Unexamined Patent Publication No. 2010-25031. The engine disclosed in Japanese Unexamined Patent Publication No. 2010-25031 includes a cylinder, an intake passage and an exhaust passage connected to the cylinder, an injector configured to inject fuel to a downstream end (an intake port) of the intake passage, a reforming chamber formed at a midway of the exhaust passage, a reforming catalyst disposed in the reforming chamber, an exhaust gas inlet passage for connecting between the reforming chamber and the exhaust passage on the upstream side of the reforming chamber, a reformed gas inlet pipe for connecting between the reforming chamber and the intake passage, and a water injection nozzle and a fuel supply device mounted to the exhaust gas inlet passage. Water injected from the water injection nozzle and fuel supplied from the fuel supply device react each other while undergoing an endothermic reaction during passing through the reforming chamber (the reforming catalyst), and turn to hydrogen and carbon monoxide (a steam reforming reaction). Reformed fuel containing hydrogen and carbon monoxide, in other words, reformed gas is introduced into the intake passage through the reformed gas inlet pipe, and is introduced into the cylinder (the combustion chamber) after being mixed with fuel injected from the injector.
Reformed gas containing hydrogen and carbon monoxide has a high lean limit, as compared with fuel before reforming, and a calorific value of reformed gas is high. Therefore, an effect of improving fuel economy of an engine is expected by introducing fuel containing reformed gas into a cylinder for combustion.
In the engine disclosed in Japanese Unexamined Patent Publication No. 2010-25031, however, fuel is reformed in the reforming chamber formed at a midway of the exhaust passage, and reformed fuel, in other words, reformed gas is introduced into the cylinder through the reformed gas inlet pipe. This may make it difficult to introduce the entirety of reformed gas into the cylinder. For instance, a part of reformed gas may flow into another path (an exhaust passage on the downstream side of the reforming chamber) other than the introduction path to the cylinder, or unreformed fuel may adhere and remain in the exhaust passage or on an inner wall of the reforming chamber. When the aforementioned condition occurs, only a part of reformed gas is used for combustion. This may offset the effect by reforming fuel, and may make it difficult to obtain a sufficient effect of improving fuel economy.